1. The Field of the Invention
The present invention relates to packaging of microelectronic components. More particularly, the present invention relates to improved dissipation of heat produced by microelectronic components. Additionally, the present invention relates to balancing a chip package against warpage stresses. In particular, the present invention relates to a unitary heat sink that functions as both as a die-attach paddle and an outer ring of a chip package for heat rejection that extends to the outer boundary of the chip package. In the unitary heat sink, at least one opening therein exists between the die-attach paddle and the outer boundary of the chip package.
2. Description of Prior Art
Microelectronic packages are routinely packaged in plastic molding compounds in order to reduce cost and facilitate packaging operations. Because microelectronic devices produce appreciable amounts of heat that must be removed from the device in order to ensure proper function thereof, the issue of heat removal has become increasingly important. Where the power rating of the microelectronic device becomes significantly high, plastic encapsulation usually will be replaced with either ceramic or metallic encapsulation to facilitate heat rejection. However, ceramic or metallic encapsulation is more expensive than plastic encapsulation.
A heat sink can be attached directly to a die to serve a dual function of a die-attach paddle and a heat sink. Conventional techniques have been developed to rigidly attach a heat sink to a lead frame for an integrated circuit. Several problems occur in chip packaging. One problem that occurs in using a composite heat sink is that disparities between coefficients of thermal expansion can cause destructive stresses in a package as the package cycles through heating and cooling.
As chip packaging technology follows the lead of microelectronic circuit design miniaturization, a goal is to miniaturize the chip package size to be substantially the same size as the chip itself. Lead on chip (LOC) technology allows leads to come directly to the microelectronic device without the need for wire bonding, thus, the chip package can have a smaller dimension.
FIGS. 1a and 1b illustrate a prior art method of heat management in which at least two problems exist. In FIG. 1a, a semiconductor package 10, depicted in a top plan view, illustrates LOC technology where a lead 12 makes direct contact to a chip 14 without a bonding wire. When viewed in the cross-section view of FIG. 1b, package 10 reveals a die attach 16 and outer structure 18 that are substantially parallel to each other and that are connected by any of various methods such as welding or adhesive bonding. As package 10 runs through thermal cycles, because of dissimilarities in coefficients of thermal expansion between die attach 16, adhesive bonding, if any, and outer structure 18, destructive stresses are caused. It can also be seen that die attach 16, outer structure 18, and any adhesive bonding material therebetween encapsulated in a packaging plastic 26 may have antagonistic abilities to withstand thermal stresses. A first thickness 20 of packaging plastic 26 above chip 14, and a second thickness 22 of packaging plastic 26 below die attach 16 may cause package 10 to be thermally unbalanced such that warpage and bowing may occur while in use. Although conductive heat flow into outer structure 18 may be substantial at the interface between die attach 16 and outer structure 18, ultimate heat rejection from package 10 is poor because outer structure 18 is substantially encapsulated in packaging plastic 26 and packaging plastic 26 acts as a heat flow resistor.
FIGS. 2a and 2b illustrate prior art attempts to substantially equilibrate dissimilar stresses in packaging plastic 26 by making first thickness 20 and second thickness 22 substantially similar. Die attach 16 is downset away from outer structure 18 by a downset 24 such that first thickness 20 and second thickness 22 are substantially similar. It can be seen, however, that a second heat sink 34, substantially externally exposed at the package lower edge 28, is insulated from die attach 16 and outer structure 18 by packaging plastic 26. Packaging plastic 26 acts as a thermal blanket and resists heat removal from chip 14. Packaging plastic 26 may be required to be injected from at least two separate injection ports when a package such as package 10 depicted in either of FIGS. 1b and 2b is being assembled. Because die attach 16 and outer structure 18 may substantially seal packaging plastic 26 in the region that forms first thickness 20 from packaging plastic 26 in the region that form second thickness 22, a dual-pressurized, dual injection-port plastic injection molding system may be needed to properly complete formation of the body of package 10.
FIG. 2c illustrates a cross-sectional view of an attempt to facilitate heat removal from chip 14 in which substantial downsetting of chip 14 to second heat sink 34 at package lower edge 28 is done wherein downset 24 extends downwardly substantially from the center line of package 10 to package lower edge 28. One problem with this structure is that first thickness 20 of packaging plastic 26 causes substantial unbalance such that warpage and bowing of package 10 is caused when package 10 is heated during ordinary use. Additionally, a two- or three-input injection-port molding technique is required.
What is needed in the art is a heat management structure that effectively conveys heat away from a microelectronic device without the problems that arise in the prior art.
More particularly, what is needed is a heat management structure that effectively rejects heat from a microelectronic device that avoids thermal stresses caused by dissimilarities in thermal conductivities of materials. What is also needed in the art is a method of forming a chip package comprising packaging plastic that simplifies injection molding techniques over the prior art. What is also needed in the art is a heat management structure that does not cause an unbalanced package to be formed such that warpage and bowing are substantially avoided.